The work of many investigators has made it clear that 90 per cent or more of the iodine in the normal thyroid gland is present in organic combination. This organic iodine, according to Harington (1933), can be completely accounted for by thyroxine and diiodotyrosine. These two compounds do not normally exist free in the gland but can be isolated after suitable hydrolysis of the thyroid protein. Various values have heretofore been reported for the proportions of thyroxine and diiodotyrosine present in thyroid tissue. Thus, by an acid separation, Harington and Randall (1929) obtained values of 40–50 for the ratio of thyroxine to total iodine. Similar values were found by others who employed their method (Rotter and Soos, 1933; Dey, 1945). Subsequently, Leland and Foster (1932), who used butyl alcohol for the extraction of thyroxine from an alkaline hydrolysate of the thyroid, reported that only 25 per cent of the iodine of the gland is present as thyroxine.